Rename custom.scss to _styles_project.scss

This is how docsy expects custom styles to be defined

assets/scss/_styles_project.scss

@@ -1,5 +1,5 @@
 /*
-Copyright 2020, 2021 The Matrix.org Foundation C.I.C.
+Copyright 2020, 2021, 2023 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -18,9 +18,6 @@ limitations under the License.
 Custom SCSS for the Matrix spec
 */
 
-@import "variables_project";
-@import "variables";
-
 /* Import the CSS classes for the syntax highlighter.
  *
  * This is generated with:
@@ -516,4 +513,4 @@ dd {
     background-color: rgba($black, 0.05);
     border-radius: 0.25rem;  // was $border-radius, but that var isn't accessible here.
   }
-}
+}

layouts/partials/hooks/head-end.html

@@ -16,13 +16,4 @@
 <link rel="preload" href="{{ "/css/fonts/Inter.css" | relURL }}" as="style">
 <link rel="stylesheet" href="{{ "/css/fonts/Inter.css" | relURL }}">
 
-{{ $scss := "scss/custom.scss"}}
+{{/* Note the missing postCSS. This makes it snappier to develop in Chrome, but makes it look sub-optimal in other browsers. */}}
-{{ if .Site.IsServer }}
-{{/* Note the missing postCSS. This makes it snappier to develop in Chrome, but makes it look sub-optimal in other browsers. */}}
-{{ $css := resources.Get $scss | toCSS (dict "enableSourceMap" true) }}
-<link href="{{ $css.RelPermalink }}" rel="stylesheet">
-{{ else }}
-{{ $css := resources.Get $scss | toCSS (dict "enableSourceMap" false) | postCSS | minify | fingerprint }}
-<link rel="preload" href="{{ $css.RelPermalink }}" as="style">
-<link href="{{ $css.RelPermalink }}" rel="stylesheet" integrity="{{ $css.Data.integrity }}">
-{{ end }}